An image acquisition or recording system (hereinafter, imaging system) is often used to acquire an image of a selected plurality of subjects located in a subject field. The subject plurality will typically include a principal subject that is located at a position with respect to other (secondary) subjects. The subject field height is defined by the subjects at the upper and lower extremes of the image field. It is often a goal of the operator of the imaging system to acquire an image of the subject plurality such that not only a principal subject but also one or more of the secondary subjects may be reproduced sharply. Consequently, autofocussing systems have been devised to set the focus of the imaging system so that the primary subject within the subject plurality is focussed adequately. In practice, however, some secondary subject positions in the upper and lower portions of the image field are such that they are not adequately focussed or are not within the field of view of the autofocussing system.
Accordingly, there have been proposed automatic focusing devices that perform focus detection with respect to a plurality of divided areas of the subject field, and the focusing condition of a continuous-focus optical assembly is controlled in accordance with the plurality of focus detection determinations.
An autofocussing system can be categorized according to whether it focusses through an image-taking lens system or through an independent optical system. In particular, one system is known in the art as a through-the-lens (TTL) phase-correlation autofocus system; one exemplary construction of the optical system used in a TTL autofocus system is shown in FIG. 1. First and second subject images are passed through an objective lens 12 at respective first and second portions distant from the optical axis 13. At a position equivalent to a predetermined focal plane 14 of the objective lens 12, there is disposed an autofocus module which may include a condenser lens 16, a pair 17 of image-forming lenses 18 and 20, and a linear array 21 of photoelectric conversion devices. The array 21 includes sections 22 and 24 disposed on the image-forming planes of image-forming lenses 18 and 20. Array sections 22 and 24 are respectively composed, for example, of first and second pluralities of photo diode cells a.sub.1 -a.sub.10. . . , and b.sub.1 -b.sub.16. . . . The output of each cell in array 21 section 22 is provided to a correlation system 28 to be sequentially converted to a digital signal D(x) (x=1, 2,3,4,5, . . . ). This image signal data D(x) may then be provided to other means (not shown) for AF processing.
A side view of the optical system of FIG. 1 is shown in FIG. 2(a), with the condenser lens 16 omitted for clarity. The separation lenses 17 are located laterally adjacent from the optical axis 13 and the linear array 21 is located on the optical axis 13 with its longitudinal axis parallel to the disposition of the separation lenses. As illustrated by the representation of the image field 22, the line sensor location corresponds to a central portion of the image field, so as to adequately sense separation images of a subject centrally located (for example, at point P1) in the image field.
Illustrated in FIG. 2(b) is a simplified representation of the conventional approach to effecting autofocussing of a subject located at a point vertically displaced from the optical axis (for example, at point P2). A second detection system including paired separation lenses 17A and linear array 21A are located at positions commensurately displaced from the optical axis 13. As illustrated again by the representation of the image field 22, the location of line sensor 25 corresponds to a vertically-displaced portion 26, so as to adequately sense separation images of the subject located at point P2.
Rather than include additional separation lens pair 24 and line sensor 25 combinations, an alternative approach is to greatly enlarge the central lens pair 18, 20 and provide an area array in lieu of the linear array 21 of FIG. 2(a).
A significant disadvantage of the first of the above-described approaches is that an undesirably large and/or complex arrangement of detection systems must be constructed to provide adequate autofocussing of the various subjects that may be present at locations displaced vertically from the optical axis. Further, the second approach (which attempts to enlarge the optical elements and provide an area array) is generally impractical due to the constraints imposed by basic optical design considerations and by the cost of fabrication of large-scale electro-optical sensors.
There is accordingly a need for an improved autofocussing system that can accurately focus one or more subjects displaced from the central portion of the image field, without the disadvantages found in the methods and apparatus of the prior art.